Radio receiving system



July 6, 1943. R. E. SPENCER RADIO RECEIVING SYSTEM Filed Dec. 12, 1935 Patented July 6, 1943 RADIG RECEIVING SYSTEM Rolf Edmund Spencer, London, England, assignor to Electric & Musical Industries, Limited, Hayes, England, a company of Great Britain Application December 12, 1935, Serial No. 54,003 In Great Britain December 13, 1934 15 Claims.

The present invention relates to wireless and like receivers for the reception of signals in the form of a modulated carrier.

Receivers of this kind are usually provided with selective tuning means to enable an operator to select a desired signal from a number of signals which are adjacent one another in the operating waveband. When the desired signal is a transmission at a wavelength within a broadcast waveband, it is necessary that the discrimination of the receiver in favour of the desired signal, that is to say, the selectivity of the receiver, shall be high; this is because of the larg number of broadcast stations usually transmitting on carriers of wavelength within the restricted compass of the broadcast wavebands.

In a receiver of high selectivity, it is found that considerable distortion may be experienced if the receiver is not accurately tuned to the desired carrier; this distortion may be due to what is known as side-band cutting, and to other causes. Considerable care on the part of an operator in tuning such a receiver is accordingly necessary if such distortion is to be avoided.

Furthermore, if it is desired to arrange that the tuning of the receiver may be controlled from a point remote from the receiver, the remote control means must be capable of providing very accurate adjustment. It is for this reason that remote control systems for wireless receivers have usually been of complicated construction and of considerable cost.

It will be clear that a need exists for means whereby the necessity for highly critical tuning of a wireless receiver may be avoided; such means would simplify the operation of tuning both at the receiver and from a remote point. In order to meet this need, it has been proposed to provide what is known as pull-in tuning, that is to say, it has been proposed to arrange the tuning system so that when the receiver is tuned approximately to a desired station, automatic means come into operation to pull the receiver exactly into tune.

It is an object of this invention to provide new or improved means for effecting pull-in tuning in a wireless receiver.

The present invention accordingly provides a wireless or like receiver having a tuning circuit, and, associated with said circuit, controlling means adapted to determine the tuning thereof and thus to compensate automatically for a measure of mis-tuning of said circuit, said means comprising a thermionic valve arranged to operate as a variable controlling impedance, where in there are provided means for varying the sensitivity of said valve in accordance with the operating frequency so that, for a pro-determined amount of mistuning, the magnitude of said controlling impedance when said receiver is arranged for the reception of a signal of W frequency is dilferent from the magnitude thereof when the receiver is arranged for the reception of a signal of a higher frequency.

According to a feature of the present invention, in a wireless or like receiver comprising means for developing a potential difference, which, in its sign and magnitude, is dependent on the sense in which, and the extent to which, the receiver is off-tune with respect to a desired signal, there is provided a thermionic valve having two control grids which are adapted to control the electron stream from the cathode to the anode thereof in succession, and means for establishing said potential difference between one of said control grids and said cathode, the arrangement being made such that the tuning of said receiver, or of a part thereof, is dependent upon the efiective reactive impedance between the other control grid and said cathode.

The invention further provides a wireless or like receiver comprising a thermionic valve having two control grids which are adapted to control the electron stream from the cathode to the anode thereof in succession, means for establishing between one of said control grids and the cathode a potential difierence which, in its sign and magnitude, is dependent upon the sense in which and the extent to Which the receiver is off tune with respect to a desired signal, and means for causing the effective reactive impedance between the other control grid and the cathode to control the tuning of said receiver or of a part thereof, characterised by the fact that there are provided means for varying the magnitude of said reactive impedance corresponding to a certain degree of mistuning in accordance with the frequency of said desiredsignal, so that pull-in tuning of substantially constant ciliciency is obtained at all frequencies within a predetermined range.

The thermionic valve referred to in the two preceding paragraphs preferably has a non-reactive load, such as a resistance, connected in its anode circuit; the grid-cathode circuit of the control grid other than that to which the potential difference referred to is applied is connected in parallel with a tuning condenser of the receiver, and it is found that in such an arrangement, there appears in the grid-cathode circuit in question a reflected capacity dependent upon the magnitude of th capacity between the anode and that control grid and upon the voltage amplification of the valve; this reflected capacity is effectively in shunt across the tuning condenser, and the tuning of the receiver is thus dependent upon the magnitude of the reflected capacity. In the case of a superheterodyne receiver, the reflected capacity is conveniently arranged to be in shunt with the tun-' ing condenser of the local oscillator.

The reflected capacity may be varied by varying the voltage amplification of the valve, and this in turn depends upon the potential diiference developed when the receiver is off-tune. Hence, as will be more fully explained hereinafter, by making the potential of a control grid of a valve of the kind referred to dependent upon the sense in which and the extent to which the receiver is ofi-tune with respect to a desired signal, the tuning of the receiver may be made to adjust itself automatically to tend to compensate for a certain degree of mistuning; that is to say, a measure of pull-in tuning may be effected.

In the case of a valve employed, in a manner such as that outlined, as a variable capacity, in which the tuning of the receiver is made to depend upcn the magnitude of the reflected capacity appearing in a control grid circuit of the valve, the reflected capacity may be augmented by the provision of a condenser connected between the anode of the valve and the control grid in the circuit of which the reflected capacity appears; according to another feature of the present invention, the magnitude of this condenser is arranged to be adjustable, the adjustment being made to depend upon the frequency to which the receiver is tuned.

Furthermore, when the valve has a resistive load connected in its anode circuit, the value of the resistance may be made adjustable, the adjustment being made dependent on the tuning of the receiver.

Instead of an adjustable condenser or resistance, a number of condensers or resistances of difierent values associated with a selector switch may be employed. In a preferred arrangement according to the invention, a variable resistance and two fixed condensers associated with a switch are provided, the variable resistance being ganged to the tuning control, whilst the switch is associated with the waveband-changing mechanism of the receiver.

The present invention further provides a wireless or like receiver comprising a tuning circuit, and. associated with said circuit, controlling means adapted under the control of an electrical eil'eot which, in its sign and magnitude, is dependent upon the sense in which and the extent to which the receiver is oil tune with respect to a desired signal, to determine the tuning of said circuit and thus to compensate automatically for a measure of mistuning thereof, in which there are provided discriminating means adapted to generate said electrical effect, characterised in that said discriminating means comprise a reactive circuit having a positive reactance, a reactive circuit having, when said receiver is accurately tuned for the reception of said desired signal, a negative reactance of substantially the same magnitude, and means for feeding radiofrequency oscillations derived from said desired signal to said two reactive circuits.

In a preferred simple arrangement, the discriminating circuit comprises a condenser in series with an inductance coil; in the case of a superheterodyne receiver, the discriminating circuit may be fed with intermediate frequency oscillations.

A preferred method of carrying the invention into effect will now be described by way of example with reference to the accompanying drawing, in which is shown a superheterodyne receiver according to the invention.

Referring to the drawing, a superheterody-ne receiver comprises a heptode valve l which acts 2 local oscillator and mixer. The first control grid, that is, the inner grid of the heptode, is connected through a leak resistance 2 shunted by a grid condenser 3, and through a parallel resonant circuit 4, to the chassis, which is indicated by the line 5. An electrode 8, which serves as an anode for the generation of local oscillations, is connected through a coil 1 to a point 8 in a source B1 of anode current; the negative terminal of the source B1 is returned to the chassis. The coil 1 is coupled to the coil 9 of resonant circuit 4, and the variable condenser I!) of that circuit is ganged in well-known manner to further variable condensers in the signal frequency stage H of th receiver, so that only one tuning control TC is necessary.

The second control grid of the heptode I is connected to the signal frequency stage I l of the receiver, the latter being associated with an antenna-earth system AB.

The coil 9 of circuit 4 is associated with a switch S1 which serves, under the control of wave-range control member WCM, to short-circuit a part of coil 9 when the receiver is required to operate within the shorter of alternative wavebands; control member WCM is also arranged to control coil switching in signal frequency stage I I,

The anode circuit of th heptode l comprises a parallel resonant circuit l2 tuned to the intermediate frequency; a coil I3 is coupled to the coil i l of tuned circuit l2, and is connected between the control grid and cathode of an intermediate frequency amplifying valve IS. The cathode of this valve is connected to the chassis through a biasing resistance [6 shunted by a bypass condenser H, and its anode is connected through an intermediate-frequency choke coil 68 to a point in the anode current source B1 at a suitable positive potential.

The anode of the valve [5 is connected to the chassis through a series resonant circuit comprising a coil 59 and a condenser 28; the latter is preferably made adjustable. Two further coils 2! and 22 are coupled to the coil l9, and the coil 22 is employed for feeding intermediate frequency oscillations from amplifier 15 to a further intermediate frequency amplifier, a second detector and output stage, referred to on the drawing as the output stage; the latter feeds a loudspeaker LS. The values of the circuit elements I9, 20, 2! and 22 are made such that the potential difieronce set up at the exact intermediate frequency across coil 21 is equal to that set up across condenser 20 at that frequency.

The junction point of condenser 20 and coil i9 is connected through a condenser 31 to the anode 23 of a double-diode valve 25 having a cathode 24, and the coil 2! is connected between the anode 26 and cathode 2A of valve 25. The diode 23, 24 has a load resistance 21 of a high value such as 5 megohms for example, and diode 26, 2G has a load resistance 28 of substantially the same high value. The load resistances 21 and 28 are shunted by by-pass condensers 29 and 30 respectively, Which may have a capacity of 0.01 microfarad. An intermediate frequency choke coil 3! is connected in shunt with diode 23, 24 and resistance 21 to complete the external direct current circuit of diode 23, 24.

The end A of load resistance 2'! remote from the cathode 24 of valve 25 is connected to the outer control grid of a hexode valve 32, having its anode connected through a resistance 33 to the positive terminal 'of an anode current source B2.

The end A of the resistance 23 remote from the cathode 24 of valve 25 is connected to the chassis 5, and the cathode of hexode 32 is connected through a biasing resistance 34 shunted by a bypass condenser 35 to the chassis.

The inner control grid of hexode 32 is connected through a condenser 38, to that side of condenser I which is not directly connected to the chassis. A grid leak 39 is connected as shown. In a hexode valve, the anode-to-inner-' grid capacity, and hence the reflected capacity in the inner grid-cathode circuit, is normally very small, and a condenser 36 is accordingly connected between the anode of hexode 32 and the inner grid thereof so as to reinforce both these capacities. Condenser 36 is shunted by a further condenser 36 which is in series with switch S2, the latter being arranged to be opened, by control member WCM, when switch S1 is closed.

The operation of the arrangement described is as follows:

When a signal is accurately tuned in, the potential differences applied to diodes 23, 24 and 26, 24 are substantially equal, as already explained, and there is thus substantially no potential difference between the points A and A'. The biasing potential difference between the outer control grid and the cathode of the hexode is thus determined by the voltage drop across the resistance 34.

Hexode valves have the following important porperty: the mutual conductance from, say, the inner grid to the anode may be varied by varying the bias on the outer control grid, and since variations in mutual conductance are accompanied by corresponding changes in amplification factor, the amplification factor with respect to the inner grid maybe varied by varying the outer control grid-cathode potential difference, for example.

If now the receiver is tuned only approximately to the signal, so that the actual frequency to which the receiver is tuned is slightly lower than the signal frequency, the resulting intermediate frequency will be, say, less than the value corresponding to a condition of precise tuning, assuming that the local oscillation frequency is always higher than the signal frequency. In these circumstances, the potential difference set up across condenser 20 is larger than that set up across coil 2|, and more current flows in diode 23, 24 than flows in diode 26, 24; point A thus becomes more negative than point A, and the outer control grid bias of hexode 32 increases in the negative sense.

It should be noted that the diodes 23, 24 and 26, 24 are provided with very high load resistances. Thus the potential differences set up across the load resistances are very nearly equal to the peak voltages applied to the diodes, the losses in the latter being negligibly small. It is thus not essential that the two diodes shall be accurately matched, since the rectified voltage in each case is substantially independent of the characteristics of the diode.

When point A becomes negative relative to point A, the amplification factor of the hexode is decreased, and the reflected capacity is also decreased. Since the reflected capacity is in shunt with the tuning condenser [0 of the local oscillator circuit 4, the frequency of the local oscillation is thus increased, the intermediate frequency is correspondingly increased, and the potential difference between points A and A is reduced. The effect continues, and the potential difference between points A and A approaches zero; as has been pointed out, when the potential difi'erence is equal to zero the intermediate frequency has its correct value, and the receiver may be regarded as being in tune, although in fact the signal frequency circuits remain slightly mistuned.

If on the other hand the receiver is mistuned to a frequency slightly above the signal frequency, the potential difference set up across coil 2| is greater than that set up across condenser 20, and the potential of point A thus becomes more positive than that of point A. The magnification factor of hexode 32, and hence the reflected capacity, is thus increased, and the local oscillation frequency is therefore decreased.

It will be noted that the change in oscillator tuning capacity corresponding to a certain degree of mistuning is a larger fraction of the oscillator tuning capacity when the receiver is tuned to a station at the short wave end of the mediumwave broadcast band, for example, than when the receiver is tuned to a station of relatively longer wavelength, since in the former case the oscillator tuning capacity is less than in the latter case.

The efiiciency with which the pull-in tuning operates is therefore greater when receiving stations of short wavelength than when receiving relatively long wave stations, and means are provided for obviating or reducing this variation in efficiency of pull-in tuning by varying the sensitivity of the valve, which forms part of the pullin tuning control means, in accordance with the operating frequency.

It has been pointed out that the anode to inner grid capacity of hexode 32 is supplemented by condenser 36 over the shorter waveband and by condensers 36 and 36' in shunt over the longer waveband, so that an adequate reflected capacity may be obtained over both wavebands.

A way of achieving a substantially continuous variation of sensitivity, is to arrange that the resistance 33 has a larger value when the receiver is tuned to a long wavelength than when it is tuned to a short wavelength. In the arrangement illustrated, resistance 33 is made adjustable, the adjusting member being ganged to tuning control member TC.

In the preferred embodiment of the invention, as applied to a receiver for operating over two different wavebands, which is illustrated in the drawing, a combination of the two methods is employed; resistance 33, which i made variable, and is ganged with condenser 10 and the tuning condensers of the signal frequency stage II of the receiver, has a maximum value of about 16,000 ohms when the receiver is tuned to the longest wavelength to be received, and the low of its variation is arranged to be approximately cubic. Condenser 36 is given a capacity of 0.5 micromicrofarad and condenser 36, a capacity of 1.5 micro-microfarads. As has been stated, a switch S2 is provided for connecting the second condenser 36 in shunt with the first, 36, and the switch S2 is ganged to the waverange-changing member WCM, so that the effective capacity for the shorter waveband is 0.5 micro-microfarad, while the effective capacity for the longer waveband is 2.0 micro-microfarads.

In the receiver described, the intermediate frequency amplifying valve l5 has been employed to feed intermediate frequency oscillations to the later stages of the receiver; in a further more elaborate receiver, however, the valve I5 is preferably a separate amplifier, and does not form a part of the amplifying chain of the receiver.

The invention has been described as applied to a superheterodyne receiver, but it is to be understood that it is not so limited. However, the application of pull-in tuning to a superheter'ody'ne receiver is less complicated than its application to so-called straight receivers, since in the case of the superheterodyne, it is only necessary to control the tuning of one tuned circuit, namely the circuit controlling the frequency "of the local oscillation; furthermore, the di criminating circuit, which de ermines the sense in which compensation for mistuning is appli d (and in the case of the receiver described is the circuit comprising condenser 28 and coil I9) may be fixedly tuned and fed with oscillations at the intermediate frequency of the receiver.

If desired, the receiver described may be modified by arranging that compensation for mistuning is also applied to the signal frequency circuits.

Instead of a valve arranged, in a manner such as that described, to simulate a variable capacity, a valve arranged to simulate a variable impedance of any other suitable kind ma be employed. Other modifications of the invention will readily occur to those versed in the art.

I claim:

1. A modulated carrier wave receiver comprising a tuning circuit, controlling means for deter mining the tuning of said tuning circuit, discriminating means operating under the control of oscillations derived from a received carrier to generate an electrical effect which, in its signand magnitude, is dependent upon the sense-and the magnitude of the mistuning of said receiver with respect to said desired carrier and means for causing said controlling means to be operated automatically under the control of said electrical effect to tend to compensate for said mistuning, said discriminating means comprising a first reactive circuit having a positive reactance, a second reactive circuit having, when said receiver is accurately tuned for the reception of said desired carrier, a negative reactance of substantially the same magnitude, said two reactiv circuits consisting of a pair of opposed rectifiers of the diode type provided with a common input circuit, a condenser and an inductance being arranged in series relation, and in resonance with the frequency of said derived oscillations, to provide said common input circuit, and means for feeding said oscillations to said first and second reactive circuits.

2. A modulated carrier wave receiver comprising discriminating means for developing a potential difference which, in its sign and magnitude, is dependent upon the sense and magnitude of the mistuning of said receiver with respect to a desired carrier, said discriminating means comprising opposed rectifiers having a wave input circuit, said input circuit consisting of a capacity and inductance serie resonant to a predetermined carrier frequency, a thermionic valve having a cathode and two control grids arranged to control the space current in said valve in succession, means for establishing said potential difierence between one of said control grids and said cathode, and means for causing the effective reactive impedance between the other of said control grids and said cathode to control the tuning of at least a part of said receiver.

3. A modulated carrier wave receiver comprising discriminating means including a discriminating circuit comprising first and second reactive circuits, means for feeding oscillations derived --frm a desired carrier to said reactive circuits,

rectifying means for deriving first and second potential differences from said first and second reactive circuits respectively, said rectifying means comprising opposed diodes each provided with a separate load resistor of such high resistance that said potential differences are very nearly equal to the peak value of the oscillations applied to the diodes, the magnitudes of said potential differences being substantially equal when said receiver is accurately tuned for the reception of said desired carrier and varying in opposite senses when the condition of accurate tuning is departed from, a thermionic valve having a cathode and two control grids arranged to control the space current in said valve in succession, means for establishing said first and second potential differences, in opposition, between one of said control grids and said cathode, and means for causing the efi'ective reactive impedance between the other of said control grids and said cathode to control the tuning of at least a part of said receiver.

4. A modulated carrier wave receiver according to claim 3, in which said first reactive circuit is constituted by a condenser, and said second reactive circuit comprises an inductance coil and said condenser and coil being series resonant to a predetermined carrier frequency.

5. A modulated carrier Wave receiver comprising discriminating means for developing a potential difference which, in its sign and magnitude, is dependent upon the sense and magnitude of the rnistuning of said receiver with respect to a desired carrier, a thermionic valve having a cathode, an anode and two control grids arranged to control the space current in said valve in succession, a resistive load connected in the anode circuit of said valve, means for establishing said potential difference between one of said control grids and said cathode, and mean for causing the effective reactive impedance between the other of said control grids and said cathode to control the tuning of at least a part of said receiver and means for adjusting the value of said load to provide a control over the eilect of said reactive impedance.

6. A modulated carrier wave receiver comprising discriminatingmeans for developing a potential difference which, in its sign and magnitude, is dependent upon the sense and magnitude of the mistuning of said receiver with respect to a desired carrier, a thermionic valve having a cathode, an anode and two control grids arranged to control the space current in said valve in succession, means for establishing said potential difference between one of said-control grids and said cathode, a connection including acondenser between said anode and the other control grid, and

means for causing the eifective reactive impedance between said other control grid and said cathode to control the tuning of at least a part of said receiver and means for providing a control over the effect of said reactive impedance.

'7. A modulated carrier wave receiver having a tuning circuit, controlling means associated with said circuit for controlling the tuning thereof and for thus compensating for a measure of mistuningof said circuit, a thermionic valve operating automatically under the control of oscillations derived from a desired carrier as a variable controlling impedance and forming part of said controlling means, and means for varying the sensitivity of said valve in accordance with the frequency of said desired carrier so that, for a .pre-determined amount of mistuning, the magnitude of said controlling impedance when said receiver is arranged for the reception of a carrier of a low frequency is different from the magnitude thereof when the receiver is arranged for the reception of a carrier of a relatively high fre quency.

8. A modulated carrier wave receiver according to claim 7, in which means are provided for varying the mutual conductance of said valve in accordance with the sense and magnitude of said mistuning.

9. A modulated carrier wave receiver according to claim '7, in which there are provided means for developing a potential difference which, in its sign and magnitude, is a measure of the sense and magnitude of said mistuning, and means for causing said potential difference to determine the magnitude of said controlling impedance.

10. A modulated carrier wave receiver comprising discriminating means for developing a potential difference which, in its sign and magnitude, is dependent upon the sense and magnitude of the mistuning of said receiver with respect to a desired carrier, a thermionic valve having a cathode and two control grids arranged to control the space current in said valve in succession,

means for establishing said potential difference between one of said control grids and said cathode, means for causing the effective reactive impedance between the other of said control grids and said cathode to control the tuning of at least a part of said receiver, and compensating means for varying the magnitude of said effective reactive impedance corresponding to a fixed amount of mistuning in accordance with the frequency of said desired carrier so that pull-in tuning of substantially constant effectiveness is obtained at all frequencies within a pre-determined range.

11. A modulated carrier wave receiver comprising tuning control means, discriminating means for developing a potential difference which, in its sign and magnitude, is dependent upon the sense and magnitude of the mistuning of said receiver with respect to a desired carrier, a thermionic Valve having a cathode, an anode and two control grids arranged to control the space current in said valve in succession, a resistive load arranged in the anode circuit of said Valve, means for establishing said potential difference between one of said control grids and said cathode, means for causing the effective reactive impedance between the other of said control grids and said cathode to control the tuning of at least a part of said receiver, and compensating means for varying the magnitude of said effective reactive impedance corresponding to a fixed amount of mistuning in accordance with the frequency of said desired carrier so that pull-in tuning of substantially constant effectiveness is obtained at all frequencies within a pre-determined range.

12. A modulated carrier wave receiver according to claim 11, in which said load is constituted by an adjustable resistance, the adjusting member being ganged to said tuning control means so that said resistance has a larger value when said receiver is arranged for the reception of a carrier of low frequency than when said receiver is arranged for the reception of a carrier of a relatively higher frequency.

13. A modulated carrier wave receiver comprising tuning control means, discriminating means for developing a potential difference which, in its sign and magnitude, is dependent upon the sense and magnitude of the mistuning of said receiver with respect to a desired carrier, a thermionic valve having a cathode, an anode and two control grids arranged to control the space current in said valve in succession, means for establishing said potential difference between one of said control grids and said cathode, a connection including a condenser between the anode and the other control grid, means for causing the effective reactive impedance between the other of said control grids and said cathode to control the tuning of at least a part of said receiver, and compensating means for Varying the magnitude of said effective reactive impedance corresponding to a fixed amount of mistuning in accordance with the frequency of said desired carrier so that pull-in tuning of substantially constant effectiveness is obtained at all frequencies within a pre-determined range.

14. In a superheterodyne receiver of the type including a local oscillator provided with a resonant tank circuit and means for varying the frequency of the tank circuit over a wide frequency range, an auxiliary frequency determining means for said tank circuit comprising a tube having connections to the tank circuit whereby a reactive effect is simulated across the tank circuit, discriminating means, responsive to oscillations derived from a received carrier, for producing a direct current voltage which is dependent in sign and magnitude upon the sense and the magnitude of the mistuning of the receiver with respect to the desired carrier, means for impressing said direct current voltage upon said tube whereby said simulating reactance tends to compensate for said mistuning, said discriminating means comprising a pair of rectifiers whose output circuits are arranged in polarity opposition to provide said voltage, a single resonant network, tuned to a predetermined operating frequency, providing a common input circuit for said rectifiers, said resonant network having its constants chosen to produce zero potential difference thereacross when the receiver is accurately tuned for the reception of the desired carrier and said derived oscillations are of said predetermined frequency, and means for feeding said derived oscillations to said resonant network.

15. In combination with the tuning circuit of the local oscillator of a superheterodyne receiver, electronic controlling means for determining the tuning of said tuning circuit, discriminating means operating under the control of oscillations derived from a received carrier to generate a direct current voltage which, in its sign and magnitude, is dependent upon the sense and the magnitude of the mistuning of the receiver with respect to the desired carrier, means for causing said controlling means to be operated automatically under the control of said voltage to tend to compensate for said mistuning, said discriminating means comprising a pair of diode rectifier circuits Whose outputs are in opposition, at least one resonant network, tuned to a predetermined operating frequency, coupled to said rectifier circuits, said resonant network having its constants chosen to produce zero potential difference thereacross when the receiver is accurately tuned for the reception of the desired carrier and said derived oscillations are of said predetermined frequency, means for feeding said derived oscillations to said resonant network and means whereby the ratio of said compensation to said mistuning is rendered substantially constant regardless of the frequency of the selected carrier.

ROLF EDMUND SPENCER. 

